The causes of high lymphocyte count in children mainly include viral infections and leukemia, as follows: 1. Viral infections: If children suffer from influenza or respiratory infections caused by respiratory syncytial virus or rhinovirus, they will have symptoms such as nasal congestion, runny nose and cough. At this time, the blood test, the blood lymphocyte count is significantly high, and the proportion of lymphocytes is also significantly high; 2, leukemia: the onset of childhood leukemia is related to genetic and acquired factors, childhood leukemia can also cause significantly high lymphocytes, and often accompanied by high fever and a significantly higher white blood cell count, you need to promptly do a bone marrow aspiration to further confirm the diagnosis.